A typical ATC radar will be using a low Pulse Repetition Frequency (i.e. number of pulses sent and recieved is low) which gives less ambiguous range resolution but is very poor at assessing Doppler shifts. This means that speed info / separation from clutter becomes harder as a consequence. This is what has happened in TIMTS case - the radar has filtered out his chopper as the radar cannot seperate his minimal doppler shift from the ground clutter at such low speeds. As he speeds up, his doppler frequency becomes detectable and his blip appears on screen.
Fighters have to trade these capabilities off against one another - as with any engineering, it's a botch!